A. Technical Field
The present disclosure relates to reducing unwanted stray light in duplex telescopes by using an apodization mask of multiwalled carbon nanotubes for light absorption.
B. Introduction
Duplex communication in telescopes is performed by using a telescope with primary and secondary mirrors to simultaneously send and receive data with a single integrated transmitter/receiver unit. This integrated unit communicates data in the form of a transmitting laser and a receiving laser which share the same axis of travel (i.e., are coaxial with one another). Upon transmission, the laser beam is expanded to fill the telescope's secondary mirror and then is reflected to the primary mirror which reshapes the laser into a collimated beam of light for transmission to another telescope. Due to imperfections in the telescope however, some of the transmitted light sent by the laser emitter may reflect off of the secondary mirror, and back into the receiver unit as stray, unwanted light. In applications where the received laser power is low relative to the transmitted laser power, this stray light may interfere with or even overwhelm reception of a weak received signal, effectively blinding the telescope's receiver with a flood of its own transmitted light. Accordingly, what is needed in the art is a system and method of efficiently and effectively suppressing the unwanted, stray transmitter light.
C. Background Science
The radiometric and imaging properties of any remote sensing instrument, such as a telescope, are tied to its within, near, and far field point spread function and stray light performance. More specifically, optical performance of remote sensing instruments depends on the diffraction and stray light suppression characteristics of that instrument and involve characterizing the multi-wavelength bidirectional scatter properties of an instrument's mirrors, apertures, baffles, vanes, and scan cavities. Observations of the Earth with remote sensing instruments are extremely challenging; its large angular extent floods scientific instruments with high flux within and adjacent to the desired field of view. This bright light diffracts from instrument structures, ricochets within the instrument and invariably contaminates measurements. Astrophysical observations are also impacted by stray light that obscures very dim objects and degrades signal to noise in spectroscopic measurements. Stray light is currently controlled by utilizing low reflectance structural surface treatments such as black paint on surfaces that reflect unwanted stray light, and by using baffles and stops to limit this background noise.
Telescopes may be used for a variety of needs, one of which is communications across great distances. Simplex laser telescopes are one form of communication device in which data is sent via laser to a receiving telescope, and the communication is one-way. Duplex telescopes, however, have transmitting and receiving elements that are unified in the same device, and allow for the simultaneous transmitting and receiving of data in the form of coaxial lasers. For application to duplex telescopes, such as in the National Aeronautics and Space Administration's planned Laser Interferometer Space Antenna (LISA), the transmitted laser may be coaxial with the received laser.
The duplex telescopes in LISA may also contain a primary and secondary mirror which may allow for a two-stage expansion of the communications signal. The primary and secondary mirror may allow for the final form of the transmitted signal upon transmission to be a collimated beam of light. The same primary and secondary mirror may allow for the final form of the received signal to be a highly concentrated beam of light that may be captured by a receiver. During transmission, the expanding transmitted beam may fill the entire secondary mirror, coaxial with the received beam, which may be directed off of the primary mirror. Duplex telescopes generally face a problem of unwanted, scattered light: since the transmitted signal is nearly on axis to the center of the secondary mirror, the transmitted light may erroneously diffract off of the secondary mirror back into the receiving photodetector as stray, unwanted light. In the case of LISA, a beam may travel the entire five million kilometer distance of space between satellites, which may greatly weaken the beam: upon arrival, the received beam may only be 100 picowatts, or one-ten-billionth the beam's original strength. The duplex telescope receiving this beam may still be transmitting its own beam, which may be one watt (i.e., nine to ten orders of magnitude higher in intensity than the weak, received signal). If any of the one watt transmitted beam is erroneously diffracted back into the photodetector, the transmitted beam may greatly interfere with or even overwhelm reception a weaker received signal.
D. Problems in the Prior Art
A problem in the prior art of duplex telescopes is stray transmitted light entering the receiver. In particular, the problem often created by duplexing is that the transmitted signal is nearly on axis to the center of the secondary mirror and may reflect some transmitted light back to the receiving photodetector. In the case of LISA, the transmitted beam may be nine orders of magnitude higher in intensity and stray transmitted light should be suppressed due its overwhelming effect when reflected directly back into the receiver. Prior art methods include both painting the affected area of the secondary mirror with a dark coating or cutting a hole into the secondary mirror to allow stray transmitter light to escape.
1. Prior Art Solution 1: Flat Black Paint
The problem of stray light interference has historically been compensated through the application of black paint on reflective areas such as mirrors, apertures, baffles, vanes and scan cavities. Such paint may include (1) LORD Aeroglaze® Z306 (Z306), (2) N-Science Corporation/Advanced Surface Technologies Optical Surfaces Deep Space Black™ (Deep Space Black) and (3) Infrared Coatings, Inc. Magic Black (Magic Black).
At grazing angles however, even the darkest paint becomes reflective, requiring the introduction of multiple baffles, stops and other means of light suppression to control stray light. In sum, black paint still allows scattering of light in telescopes, possibly preventing the performance levels needed in highly sensitive laser transmissions such as in LISA.
2. Prior Art Solution 2: Cutting a Hole into the Secondary Mirror
The problems are introduced with the alternative solution of simply cutting a hole in the secondary mirror to allow some transmitted light to escape. One problem with use of a hole in the secondary mirror is the introduction of spurious light from outside sources entering the receiver. Examples of spurious light from outside sources include bright objects such as stars, planets and other celestial bodies. When stray light from other sources is introduced, using a hole may result in interference leading to unacceptable performance. Another problem introduced by cutting a hole in the secondary mirror is the engineering challenge involved. Cutting a hole may create structural weaknesses in the secondary mirror and may cause it to crack, an unacceptable risk in space telescopes where repair may be difficult or impossible due to the inaccessibility of a space telescope. A third problem with the use of a hole in the secondary mirror is spalling around the hole (the non-uniform edge around a hole created as a natural result from drilling). Evaluation of peak irradiance shows that spalling around the edges of the hole may actually contribute additional stray transmitted light into the receiver.